Changes of glial Na+–K+ ATPase (α1 subunit) immunoreactivity in the gerbil hippocampus after transient forebrain ischemia

Coumestrol treatment prevents Na+, K+ -ATPase inhibition and affords histological neuroprotection to male rats receiving cerebral global ischemia

OBJECTIVE

In this study, we investigated the possible mechanisms underlying the neuroprotective effects of coumestrol, a potent isoflavonoid with antioxidant activities and binding affinities for both estrogen receptors (ER) ER-alpha and ER-beta that are comparable to those of 17beta-estradiol, in a model of global ischemia in male subjects.

METHODS

Wistar rats underwent global ischemia (10 minutes) or sham surgery and received a single intracerebroventricular (icv) infusion of 20 μg of coumestrol or vehicle 1 hour before ischemia or 0, 3, 6, or 24 hours after reperfusion.

RESULTS

The data analysis revealed an extensive neuronal death in the CA1 hippocampal subfield at 7 days, and a significant decrease in the Na+, K+ -ATPase activity at 1 and 24 hours after ischemia, and both injuries were attenuated by coumestrol administration.

CONCLUSIONS

Coumestrol treatment was effective in preventing neuronal loss in all times of administration as well as able to rescue the Na+, K+ -ATPase activity, suggesting its potential benefits for either prevention or therapeutics use against cerebral ischemia in males.

Expression and activity of the na-k ATPase in ischemic injury of primary cultured astrocytes

Astrocytes are reported to have critical functions in ischemic brain injury including protective effects against ischemia-induced neuronal dysfunction. Na-K ATPase maintains ionic gradients in astrocytes and is suggested as an indicator of ischemic injury in glial cells. Here, we examined the role of the Na-K ATPase in the pathologic process of ischemic injury of primary cultured astrocytes. Chemical ischemia was induced by sodium azide and glucose deprivation. Lactate dehydrogenase assays showed that the cytotoxic effect of chemical ischemia on astrocytes began to appear at 2 h of ischemia. The expression of Na-K ATPase α1 subunit protein was increased at 2 h of chemical ischemia and was decreased at 6 h of ischemia, whereas the expression of α1 subunit mRNA was not changed by chemical ischemia. Na-K ATPase activity was time-dependently decreased at 1, 3, and 6 h of chemical ischemia, whereas the enzyme activity was temporarily recovered to the control value at 2 h of chemical ischemia. Cytotoxicity at 2 h of chemical ischemia was significantly blocked by reoxygenation for 24 h following ischemia. Reoxygenation following chemical ischemia for 1 h significantly increased the activity of the Na-K ATPase, while reoxygenation following ischemia for 2 h slightly decreased the enzyme activity. These results suggest that the critical time for ischemia-induced cytotoxicity of astrocytes might be 2 h after the initiation of ischemic insult and that the increase in the expression and activity of the Na-K ATPase might play a protective role during ischemic injury of astrocytes.

Adenosine preconditions against ouabain but not against glutamate on CA1-evoked potentials in rat hippocampal slices

Hypoxic and ischaemic brain damage are believed to involve excessive release of glutamate, and recent work shows that glutamate-induced damage in brain slices can be reduced by preconditioning with hypoxia or glutamate itself. Because adenosine is a powerful preconditioning agent, we have investigated whether adenosine could precondition against glutamate in vitro. In rat hippocampal slices, glutamate depolarization reduced the amplitudes of antidromic- and orthodromic-evoked potentials, with only partial recovery. Applying adenosine before these insults failed to increase that recovery. Ouabain also produced depolarization with partial reversibility, but adenosine pretreatment increased the extent of recovery. The preconditioning effect of adenosine on ouabain responses was prevented by blocking receptors for N-methyl-D-aspartate (NMDA), but not receptors for kainate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and was blocked by inhibiting nitric oxide synthase. Preconditioning was also abolished by the ATP-dependent potassium channel blockers, glibenclamide (cytoplasmic) or 5-hydroxydecanoate (mitochondrial). We conclude that adenosine does not precondition against glutamate in hippocampal slices, but that it does precondition against ouabain with a pharmacology similar to studies in vivo. Ischaemic neuronal damage is a complex of many factors, and because adenosine can precondition against ischaemic neuronal damage, its failure to protect against glutamate highlights limitations of using glutamate alone as a model for ischaemia. Because damage following ischaemia, trauma or excitotoxicity also involves reduced Na(+),K(+)-ATPase activity, and adenosine can precondition against ouabain, we propose that ouabain-induced damage represents an additional or alternative model for the contribution to cell damage of Na(+),K(+)-ATPase loss, this being more relevant to the mechanisms of preconditioning.

Transient ischemia-induced expression and changes of tyrosine kinase A in the hippocampal dentate gyrus of the gerbil

The present study examined ischemia-related changes in tyrosine kinase A (trkA) immunoreactivity and its protein content in the dentate gyrus after 5 min of transient forebrain ischemia in gerbils. One day after ischemic insult, cresyl violet-positive polymorphic cells showed ischemic degeneration. The ischemia-induced changes in trkA immunoreactivity were found in the polymorphic layer (PL) and granule cell layer (GCL) of the dentate gyrus. In the sham-operated group, trkA immunoreactivity in the dentate gyrus was very weak. From 30 min after ischemia, trkA immunoreactivity was increased in the dentate gyrus and peaked in the dentate gyrus at 12 h after ischemia-reperfusion. Thereafter, trkA immunoreactivity was decreased time-dependently after ischemia-reperfusion. Four days after ischemic insult, trkA immunoreactivity was similar to that of the sham-operated group. In addition, it was found that ischemia-related changes in trkA protein content were similar to the immunohistochemical changes. These results suggest that the chronological changes of trkA in the dentate gyrus after transient forebrain ischemia may be associated with ischemic damage in polymorphic cells of the dentate gyrus.

Tyrosine kinase A but not phosphacan/protein tyrosine phosphatase-ζ/β immunoreactivity and protein level changes in neurons and astrocytes in the gerbil hippocampus proper after transient forebrain ischemia

In the present study, ischemia-related changes in tyrosine kinase A (trkA) and phosphacan/protein tyrosine phosphatase-zeta/beta (PTP-zeta/beta) immunoreactivities and protein contents were examined in the hippocampus proper after transient forebrain ischemia for 5 min in a gerbil model. Our investigations showed that ischemia-induced changes occurred in trkA immunoreactivity in the hippocampal CA1 region, but not in the CA2/3 region of the hippocampus proper. In the sham-operated group, trkA immunoreactivity was barely detectable. trkA immunoreactivity increased from 30 min after ischemia and peaked at 12 h. Four days after ischemic insult, trkA immunoreactivity was observed in GFAP-immunoreactive astrocytes in the strata oriens and radiatum. In addition, we found that ischemia-related changes in trkA protein content were similar to immunohistochemical changes. On the other hand, PTP-zeta/beta immunoreactivities in the hippocampus proper were unaltered by forebrain ischemia. These results suggest that chronological changes of trkA after transient forebrain ischemia may be associated with an ischemic damage compensatory mechanism in CA1 pyramidal cells.

Potassium homeostasis in the ischemic brain

Extracellular [K+] can range within 2.5-3.5 mM under normal conditions to 50-80 mM under ischemic and spreading depression events. Sustained exposure to elevated [K+]o has been shown to cause significant neuronal death even under conditions of abundant glucose supply. Astrocytes are well equipped to buffer this initial insult of elevated [K] through extensive gap junctional coupling, Na+/K+ pump activity (with associated glycogen and glycolytic potential), and endfoot siphoning capability. Their abundant energy availability and alkalinizing mechanisms help sustain Na+/K+ ATPase activity under ischemic conditions. Furthermore, passive K+ uptake mechanisms and water flux mediated through aquaporin-4 channels in endfoot processes are important energy-independent mechanisms. Unfortunately, as the length of ischemic episode is prolonged, these mechanisms increase to a point where they begin to have repercussions on other important cellular functions. Alkalinizing mechanisms induce an elevation of [Na+]i, increasing the energy demand of Na+/K+ ATPase and leading to eventual detrimental reversal of the Na+/glutamate- cotransporter and excitotoxic damage. Prolonged ischemia also results in cell swelling and activates volume regulatory processes that release excessive excitatory amino acids, further exacerbating excitotoxic injury. In the days following ischemic injury, reactive astrocytes demonstrate increased cell size and process thickness, leading to improved spatial buffering capacity in regions outside the lesion core where there is better neuronal survival. There is a substantial heterogeneity among reactive astrocytes, with some close to the lesion showing decreased buffering capacity. However, it appears that both Na+/K+ ATPase activity (along with energy production processes) as well as passive K+ uptake mechanisms are upregulated in gliotic tissue outside the lesion to enhance the above-mentioned homeostatic mechanisms.

Chronological alterations of neurofilament 150 immunoreactivity in the gerbil hippocampus and dentate gyrus after transient forebrain ischemia

In this study, we observed the chronological alterations of neurofilament 150 (NF-150) immunoreactivity in the gerbil hippocampus and dentate gyrus after 5 min transient forebrain ischemia. NF-150 immunoreactivity in the sham-operated group was mainly detected in mossy fibers and in the hilar region of the dentate gyrus. NF-150 immunoreactivity and protein contents of NF-150 and RT 97 (polyphosphorylation epitopes of neurofilament) were significantly decreased at 15 min after ischemic insult. Between 30 min and 12 h after ischemic insult, NF-150 immunoreactivity and protein content were significantly increased as compared with the sham-operated group. Thereafter, NF-150 immunoreactivity and protein content started to decrease. At 12 h after ischemic insult, unlike dentate gyrus, NF-150 immunoreactivity increased in pyramidal cells of the CA1 region. Thereafter, NF-150 immunoreactivity in the CA1 region started to decrease, and 4 days after ischemic insult, NF-150 immunoreactivity nearly was similar to that of the sham-operated group. These biphasic patterns of NF-150 immunoreactivity in the hippocampus and dentate gyrus are reverse correlated with that of the intracellular calcium influx. For calcium detection in the CA1 region, we also conducted alizarin red staining. Alizarin red positive neurons were detected in some neurons at 15-30 min after ischemic insult. At 12 h after ischemia, alizarin red positive neurons were decreased. Thereafter, alizarin red positive neurons started to decrease, but alizarin positive neurons were significantly increased in dying neurons 4 days after ischemia. These results suggest that ischemia-related changes of NF-150 expression may be caused by the calcium following transient forebrain ischemia.